The CPUs used in recent personal computers consume ever larger amounts of power owing to higher clock speeds and a greater degree of integration. For this reason, CPUs are provided with a function for conserving power by placing the CPU in a sleep mode when there is little demand for execution of processing from the system. The function that provides the sleep (or idle) mode is referred to as a "stop-clock function". In an actual personal computer, consumption of power is reduced by repeatedly alternating between the operating and sleep states.
Various techniques have been disclosed for performing the kind of control required for the stop-clock function.
By way of example, the specification of Japanese Patent Kokai Publication JP-A-9-237130 discloses a control method which includes determining the type of hardware interrupt that occurs when a CPU is in the sleep mode and forcibly restoring the CPU to the operating state depending upon the type of interrupt.
Also JP-A-8-6662 discloses a stop-clock control method which allows to place the CPU in the sleeping state when there is no input signal during a predetermined period of time and allows the user to set the state of CPU at his will.
The stop-clock function is such that when a CPU makes a transition from the sleep to the operating state as a result of this function, there is a sudden increase in the working current of the CPU. As a consequence, a voltage drop is produced as by impedance in the wiring of the power line that supplies the CPU with power, and the power-supply terminal voltage of the CPU temporarily falls below the allowable operating voltage of the CPU. This is a cause of CPU malfunction. Conversely, when the CPU undergoes a transition from the operating to the sleep state, the working current of the CPU declines sharply and the voltage across the power-supply terminals of the CPU rises.
FIG. 8 illustrates an example of the construction of a power-supply control circuit used in a conventional personal computer. Here a CPU 9, which is supplied with power from a DC power supply 1, is connected via a power-supply line 2 comprising a cable, connector and a printed circuit board, etc. The power-supply line 2 has a line impedance 4 composed of a resistance or impedance component.
The power-supply terminal voltage of the CPU 9 fluctuates when the working current of the CPU 9 fluctuates owing to the line impedance 4. In order to prevent this, a DC-DC converter 11 is disposed in the vicinity of the CPU 9 and the output side of the converter 11 is provided with a plurality of large-capacity capacitors 10 to suppress the fluctuation across the power-supply terminals of the CPU 9.